Ultrasonics in heterogeneous metal catalysis: sonochemical chemo- and enantioselective hydrogenations over supported platinum catalysts

Sonochemical chemo- and enantioselective hydrogenations over supported plat inum catalysts are described. We disclose our results with respect to a son ochemical modification of the chemoselective hydrogenation of cinnamaldehyd e over supported platinum catalysts, and the asymmetric hydrogenation of et hyl pyruvate promoted by various ultrasonic pretreatments. The ultrasonic pretreatment of the supported platinum catalysts was found t o be highly beneficial in almost every case, improving both the catalytic a ctivity and selectivity. The effect of additional experimental variables, s uch as hydrogen pressure, catalyst support, temperature and the ultrasonic insonation time were also studied. The enantioselectivity of the hydrogenat ion of ethyl pyruvate increased up to 97.1% ee. In the case of cinnamaldehy de hydrogenation, the selective preparation of cinnamyl alcohol became poss ible. The theoretical aspects of the working mechanisms in comparison with 'silent' reactions will also be provided. (C) 1999 Elsevier Science B.V. Al l rights reserved.